The present invention relates to a yarn feeding device with constant adjustable tension, particularly for feeding yarns to weaving and knitting machines and devices.
It is known that the increase in the productivity of weaving machines depends on the possibility to impart increasingly higher speeds and accelerations to the yarns being woven; on the other hand, it is also evident that this fact, as it involves higher mechanical stresses and, hence, an increased number of breaks in the warp yarns, is in contrast with the declared purpose of increasing the productivity. In fact, at each break in the warp yarn, the machine has to be stopped in order to restore, practically always by hand, the integrity of the yarn being woven.
From this point of view, the problem of feeding weaving machines with yarns unwinding at constant tension, appears in all its importance; in fact, the fundamental cause for breaks is the irregularity of unwinding tensions, due to non-constant braking and to the irregular winding of the yarns into reels or coils. It is actually due to these irregularities that the high feeding speeds, now-a-days easily reaching about 1500 meters per minute, produce tension concentrations in very reduced times; and the resulting abrupt tensions at the weakest points of the yarn cause the breaking thereof.
The object of the present invention is to provide a yarn feeding device -- to be arranged between yarn supplies, formed by reels or coils, and weaving or knitting machines or devices -- adapted to eliminate any unwinding irregularities upstream of said machines or devices, and hence allowing the same to draw the yarn in a smooth and regular way.
Devices with this object are already known: for example, the Italian Patents Nos. 659,733 and 785,283 describe a device, the essential characteristics of which lie in the presence therein of a cylindrical body which may be caused to rotate by a suitable motor, a certain reserve of yarn being tangentially wound on said cylindrical body and axially unwound therefrom, and the outlet braking of the yarn being obtained by means of elastic rings under which passes the yarn itself; electronic and electromechanical systems, adapted to start and stop the rotation of said motor, allow one to check the consistency of the yarn reserve and to restore the same, when it has decreased below certain preestablished amounts, by causing the rotation of the cylindrical body; the regular arrangement of the turns on the cylindrical body is provided for by a beveled inlet edge which, producing an axial thrust, moves forward the turns as they are formed, the task of said beveled edge being facilitated by the centrifugal force which loosens the tension of the formed turns, favouring their axial sliding along the highly smooth surfaced cylindrical body.
A device adopting the same fundamental scheme as the previous ones is that according to Italian Patent No. 795,923 which, however, in order to guarantee the regular arrangement of the turns on the cylindrical body of the device, provides for the use of an oscillating plate rotating with the cylindrical body itself and adapted to push forward the turns being formed.
These devices, however, involve fairly serious drawbacks: for example, it can easily be understood how the fact of relying on the actual yarn being wound for the formed turns to be pushed forward, is itself a remarkable cause for irregularity and crossing over of the turns; moreover, in the case of hairy yarns, the turns being formed imprison beneath them the hairs of the previous turn, which fact, during unwinding, produces sufficient tears to cause the breaking of the yarn. From this point of view, the arrangement according to Italian Patent No. 795,923 is no doubt preferable; here, the task of pushing forward the turns is entrusted to the oscillating plate, which further creates a free space in the winding area of the turns being formed, preventing the crossing over of yarns and hairs. On the other hand, according to this last patent, in order to check the amount of yarn reserve, use is made of the pressure, increasing with the number of turns, which the wound turns exert on the oscillating plate. This pressure, as it increases, changes the inclination of the plate, and this causes at a certain point the tripping of the microswitch controlling the rotation of the cylindrical body and being mechanically connected, through a sliding contact, to the plate itself; this fact is highly negative, as it means that the oscillating plate, when actually varying its inclination, no longer has, at a given moment, the strength required to overcome the friction opposing the advancing of the turns; moreover, owing to reduced oscillation, there is no space left for the new turns, and the total effect is a considerable crossing over; in addition, the amount of reserve depends on the yarn friction factor, it can by no means be adjusted and it varies from case to case. From this point of view, the arrangement of the other two cited patents is preferable, as, in such patents, the yarn reserve is checked by means of a photoelectric system which "watches" when the head of the wound reserve reaches a certain position adapted to be easily predetermined by axially displacing the photoelectric receiver. This system, however, has a double drawback: the first being that it is too expensive and that therefore, from the economic point of view, it increases unduly the total cost of the device; and the second being that it is likely not to work properly, mainly due to the dust which often develops in great quantities in the room where the device is meant to operate, and which dims the excitation lamp or dirties the reflecting surfaces of the photoelectric device.
In the same devices, a further cause of low efficiency is the outlet yarn braking system, which consists of elastic rings; this system, in the first place, allows no continuous adjustment; in fact, the elastic rings being applied on the outlet edge of the cylindrical body by no means allow a regular braking action, and a different braking effect could be obtained only by changing them into other rings of different material and rigidity. In the second place, such rings usually rotate together with the cylindrical body and, hence, the braking effects produced are substantially different, according to whether the weaving or knitting machine or device draws the yarn when the winding cylinder is or is not moving; since the winding cylinder passes from standstill to rotation just at the moment when the yarn is being drawn, the latter will undergo abrupt braking changes, which will result in tension variations.
Finally, the actual fundamental principle on which are based the devices described hereabove, i.e. that of winding a yarn reserve onto a rotating element, is criticizable from the point of view of its practical realization: because of the speeds involved, and the consequent need for the rotary element to act very promptly, the latter would have to be equipped with motors having a very high take-off power, which motors designed to equip each single feeder are not likely to provide, at least for reasons of space and expense. Hence, the feeder itself does not usually provide the characteristics of autonomy which would instead be required. Moreover, since the yarn unwound by the devices based on the above principle takes part in the dynamic condition of the cylinder on which it is wound, the starting and stopping of the feeding cylinder result in sharp tension variations of the yarn itself, which is by no means in accordance with the purposes of the devices.
Therefore, another object of the present invention is to provide a constant tension yarn feeding device, allowing one to avoid all the drawbacks typical of the aforementioned known devices, and allowing one to carry out in the most rational, efficient and safe way, the feeding of modern weaving and knitting machines.